For some years now, surgical devices, such as sutures, have been made from various synthetic absorbable materials. One example of such a synthetic absorbable suture is described in U.S. Pat. No. 3,297,033 issued Jan. 10, 1967, to Schmidt, et al., entitled "Surgical Sutures". Other examples of absorbable polymers which can be used to make surgical products are disclosed in U.S. Pat. Nos. 3,044,942, 3,371,069, 3,531,561, 3,636,956, RE 30,170 and 4,052,988.
Surgical devices such as sutures, protheses, implants and the like are usually sterilizable. In all of the prior art synthetic absorbable surgical devices, sterilizing may be accomplished by the normal use of heat or ethylene oxide sterilization or by other types of sterilization; however, it is believed that none of the prior art synthetic absorbable materials are, as a practical matter, sterilizable by radiation sterilization such as gamma radiation using a .sup.60 Co source. Some of the prior art indicates that synthetic absorbable material may be sterilized by irradiation or radiation, but we have found that radiation sterilization of the prior art synthetic absorbable materials at any practical usable level degrades the fabricated absorbable material to such an extent to render it unusable. The radiation sterilization of prior art synthetic absorbable sutures leads to distinct degradation in mechanical properties and to clinically unacceptable in vivo strength retention.
Three well accepted synthetic absorbable polymer materials which have been used to produce surgical devices, including sutures are polyglycolide, 10-90 poly(1-lactide-coglycolide) and poly-p-dioxanone. Tests have indicated that these products are only sterilizable by ethylene oxide, that radiation sterilization produces significant losses in both the physical and strength dependent biological properties of the material. These effects were discussed in an article written by Pitmann, et al. and appearing in the Journal of Polymer Science/Polymer Chemistry Edition, Volume 16, page 2722, 1978. Attempts to sterilize these polymers with more efficient and economical means, such as gamma radiation using a .sup.60 Co source, have proved impractical because of unacceptable deterioration in the tensile properties and in the in vivo performance of these polymers after gamma radiation. This is not unexpected if one recognizes the similarity in chemical structure between these polymers and the highly radiation sensitive polyoxymethylenes. Hence, the susceptibility of the molecular chains constituting these polymers are most likely to be highly radiation sensitive. Contrasted to this, poly(ethylene terephthalate), which is used to produce non-absorbable surgical devices, is readily sterilized with gamma radiation using a .sup.60 Co source without significant loss in tensile properties. This is not surprising since the aromatic nature of the polymer chain is often associated with protection against gamma radiation degradation. It is believed that the poly(ethylene terephthlate) technology and the poly(lactide) technology have not been combined in an attempt to produce a hybrid material which may be absorbable yet stable against irradiation because of the diverse manner in which these polymers are made and the lack of common catalysts that can be used effectively in both types of polymerization. Additionally, at the high temperatures required for the synthesis of poly(ethylene terephthalate) the absorbable polylactones would undergo thermal degradation. Furthermore, it was believed that incorporating aromatic sequences in an absorbable chain could compromise the desirable physical and biological properties of an absorbable polymer.
In U.S. Pat. No. 2,516,955 there are disclosed some plasticized polymers. The plasticizers disclosed are esters of p-phenylene-dioxydiacetic acid. Low molecular weight polyesters of the latter acid are claimed to have been produced by Spanagel and Carouthers as reported in their article in the Journal of American Chemical Society, Vol. 57, pp. 935-936, 1935.